Greyscale inkjet technology operates in a similar way to binary inkjet printing but has the ability to fire a range of drop sizes. The greyscale effect is achieved by firing multiple droplets from the same nozzle of a printhead in rapid succession, rather than a single droplet as occurs at binary applications. The salvo of droplets merges in flight to form inked areas, on the printing substrate, of variable size and therefore of variable greyscale and can enable the production of an enhanced image quality at equal spatial resolution.
In FIG. 1 and FIG. 2 an example of a greyscale printhead is illustrated which includes three main parts, i.e. a cover component 1 and a channel component 2, both made from lead zirconium titanate (PZT), and a nozzle plate (not shown in the figures) made from a polyimide film. PZT is a piezoelectric material, which is a material that deforms when an electric field is applied to it. By sawing parallel grooves into the PZT channel component 2, the ink channels 3 and the shared walls 4 between the ink channels 3 are being defined. Furthermore, electrodes 5, to which a voltage can be applied to eject a drop of ink out of a nozzle, are positioned at the upper parts of the shared walls 4. Finally, a cover component 1 is attached to the tops of the shared walls 4 by a thin rigid layer of glue.
Typically, in one form of printer, the printhead will be moved relative to the printing substrate to produce a so-called raster line which extends in a first direction, e.g. across a page. The first direction is sometimes called the “fast scan” direction. A raster line includes a series of sequential dots delivered onto the printing medium by the marking elements of the printhead. The printing medium is moved, usually intermittently, in a second direction perpendicular to the first direction. The second direction is often called the slow scan direction.
In FIG. 3 a general waveform that may be used to drive a channel for printing a single droplet, is illustrated. The waveform in FIG. 3 consists of 4 parts A, B, C and D. The voltage at the ordinate shows the voltage across the shared walls 4 of the printing channel, i.e. the voltage between the electrodes 5 of the printing channel and the electrodes 5 of the neighbouring channels. From the change from A to B on, the voltage at for example channel Cb is kept positive. Simultaneously, the voltage at its neighbouring channels Ca and Cc is kept low. The voltage difference between the channel Cb and its neighbouring channels Ca and Cc creates an outward bending of the shared walls 4 of channel Cb, as is shown in FIG. 1. Hence, a low pressure or vacuum occurs and ink flows into channel Cb. This phase is called the pull phase. By changing from part B to part C in the waveform as shown in FIG. 3, the shared walls 4 return in their original position, the pressure increases and a droplet is created. During the D-phase, which is also called push phase, the voltage at channel Cb is low and the voltage at its neighbouring channels Ca and Cc is positive. The shared walls 4 bend inward as is shown in FIG. 2. The deflection of the shared walls 4 of channel Cb pressurises the ink in the ink channel, ejecting the ink droplet from nozzle of channel Cb.
To create the waveform of FIG. 3 and drive a channel to eject a single droplet, a combination of the following three signals may be applied to the electrodes of the printing channel and its neighbouring channels, for example:
On (on-signal):01111111110000000000000000000000Off (off-signal):00000000000011111111111111111000Ina (inactive-signal):00000000000011111111111111111000So, for driving a single channel of the above described grayscale printhead three signals are required, i.e. an on-signal (on), an off-signal (off) and an inactive-signal (ina) (see FIG. 1 and FIG. 2). In commercially available drive circuitry for these grayscale printheads each drive signal is 480 bits long. One single little drop or droplet is determined by 32 bits. Thus, a maximum of 480/32=15 droplets can be created within one drive signal for merging together to form one big drop. If, for example, printing needs to be done with channel Cb, an on-signal is sent to channel Cb and an inactive-signal is sent on the two neighbouring channels Ca and Cc. To fire n droplets, the on-signal is sent n times to channel Cb during the first 32*n bits. During the following 32*(15-n) bits, the off-signal is sent to that channel Cb. In the same time the inactive-signal is sent to the two neighbouring channels Ca and Cc. Each of the bits takes a time of one sample-clock period. The sample clock may be chosen dependent on the drive signal required.
Present-day printers need to be fast and accurate. Very important therefore is to have control over dot size and velocity. The dot size is related to the volume of the ink drop jetted and is determined by the number of ink droplets merged within the ink drop. Furthermore, it is also important that the ink drop is exactly positioned on the printing medium, which may for example be a paper or plastic sheet, in order to reduce drop misplacement printing artefacts of which most of them get worse as printing speed increases.
In U.S. Pat. No. 5,202,659, herein incorporated by reference in its entirety for background information only, a method for operating a drop-on demand ink jet printing system in resonant mode is described for providing high resolution printing upon a recording medium. According to this document, the volume of ink droplets ejected is controllable by synchronously exciting either one or a combination of the fluidic and mechanical resonant frequencies of the ink jet apparatus. Hereby a dominant resonant frequency disturbance is produced within the associated ink chamber permitting either one of one cycle or one sub-harmonic cycle of the dominant resonant frequency to be produced. The resonant oscillations generate multiple pulses. The method described in this document is a multi-pulse method using the dominant resonant frequency of the ink jet device to produce droplets of ink of controllable volume through pulsation of a transducer at a repetition rate of the dominant resonant frequency, using either a single or a plurality of pulses at the dominant resonant frequency, dependent on the dot size required.
U.S. Pat. No. 5,202,659 provides a method of operating an ink jet device using one or a multiple number of drive pulses for operating the device over a given dot production time for producing ink droplets, each of a known volume of ink. For this, the shape and periodicity of the drive pulses utilised are carefully controlled, whereby the periodicity of the drive pulses utilised is made substantially equivalent to the dominant resonant frequency of the ink jet device.
Thus, in U.S. Pat. No. 5,202,659 droplets are fired at a rate according to the resonant frequency. A disadvantage of the method is that, for creating different droplets, always the same waveform is to be used. Further, the resonant frequency can be sensitive to small changes in the mechanical properties of the printhead, imposing high consistency requirements on printhead manufacturing. Furthermore, to keep a system in resonance, less energy is needed than to get the system in resonance. By using the same waveforms to generate first and subsequent droplets, there will be an excess of energy present in the ink chamber, possibly leading to inconsistent or uncontrollable jetting.
In U.S. Pat. No. 6,102,513, herein incorporated by reference in its entirety for background information only, a method is described for variable greyscale printing while eliminating image artefacts caused by quantisation errors, visible noise and excessive ink lay-down while also reducing printing time and improving accuracy of ink drop placement. The method in U.S. Pat. No. 6,102,513 uses timing control of electronic waveforms for variable greyscale printing (see FIG. 4). The electronic waveform may include a plurality of “square” pulses and may be characterised by a set of predetermined parameters. The parameter values for the pulse amplitudes A1, A2, etc., pulse widths W1, W2, etc., and pulse delay time intervals S1-2, S2-3, etc. between pulses are selected according to a desired mode of operating the printhead.
In the desired mode, frequencies of pulses are reinforced by the resonance frequencies of the ink chamber. Hence, the amount of energy input to the channel to cause an ink drop ejection therefrom is minimised. It is important to control the timing of the waveforms in order to eliminate variability in ink drop placement caused by differences in pulse widths or delays, and to control voltage amplitudes of the waveform in order to eliminate variability in ejection velocity of drops. To correct for this, the waveform starts with a starting delay time Ti. In this known method the printhead may for example be designed such that ink drops having different volumes are ejected at essentially the same velocity. Timing control may be done depending on the operator-selected printing mode, printing speed, receiver type and/or output image resolution.
Although the method described in U.S. Pat. No. 6,102,513 deals with a number of the problems linked with the method described in U.S. Pat. No. 5,202,659, a disadvantage of the method is that three parameters per pulse have to be determined to create a waveform, i.e. pulse width, pulse delay and pulse amplitude. Furthermore, the method uses the resonance frequency of the ink chamber. Hence, the printing rate using this method is restricted to this resonance frequency.
In U.S. Pat. No. 5,285,215, herein incorporated by reference in its entirety for background information only, a method for operating an ink jet apparatus for providing selective control within a range of the volumes of the ink drops ejected by the apparatus and/or the amount of ink striking a desired point on a recording medium, is disclosed. It is alleged that broader control of the boldness and toning of printing could be obtained by operating a transducer of an ink jet printhead in an iterative manner. This causes a plurality of successively higher, lower or equal velocity ink droplets or some combination thereof to be ejected from the nozzle of the ink jet printhead, within a time period permitting the droplets to either merge in flight prior to striking a recording medium or upon striking the recording medium at the same point.
The volume of ink striking a recording medium at a given point is thereby partly determined by the number of ink droplets merged prior to striking or at the point of striking. In the document, the velocity of the droplets is determined by changing the amplitude or changing the fall time of the trailing edge of the control pulses. If either the amplitude of the control pulse is increased, or the fall time of the trailing edge is decreased, or a combination of both, the droplets ejected from the printhead will have an increased velocity.
Using the method described in U.S. Pat. No. 5,285,215 the shapes of the waveforms used to drive the ink jet apparatus can be designed to cause successively produced ink droplets to have successively higher or lower relative velocities, or some combination thereof, so long as system timing permits the droplets to strike the recording medium at substantially the same point. A disadvantage of the method described in this document is that again, 3 parameters need to be taken into account, i.e. amplitude of each pulse, the length of each pulse and the time between every two pulses. Furthermore, the time to eject for example two droplets that merge into one drop takes about 100 μs. Therefore, it is not possible to print at very high rates, using the method described in U.S. Pat. No. 5,285,215.
U.S. Pat. No. 4,513,299, herein incorporated by reference in its entirety for background information only, describes a method for generating ink drops on demand having selectively variable size. In this method, one subvolume of ink is produced for each voltage pulse applied to the printhead (see FIG. 5). In order to produce bigger volumes of ink, a successive number N of voltage pulses is applied to the printhead within one drop production times T. The delay time between the N voltage pulses generating the individual droplets is fixed and short with respect to T and all pulses have the same shape (see FIG. 5). The N droplets merge at the position of the nozzle plate and form a drop of which the volume equals N times the volume of one droplet. In this document it is assumed that the velocity of an ink drop, including different droplets, is determined by the velocity of the first of the droplets ejected from the printhead. According to this assumption, drops including different numbers of ink droplets thus all will have the same velocity, i.e. the velocity of the first droplet that has been ejected from the print head. However, practice shows that the bigger an ink drop is, the lower the velocity of the ink drop will be because, according to Stokes law, the friction force with the air will be higher for larger drops. Therefore, in practice, drops including a different number of droplets or subvolumes of ink appear to have a different velocity. Hence, the method described in U.S. Pat. No. 4,513,299 does not allow printing of ink drops, including a different number of droplets or subvolumes of ink, with all drops having the same velocity.